1. Field of Invention
The present invention relates to inverter-type electronic ballasts for fluorescent lamps, particularly of the type using series-resonant LC circuit means for matching the inverter's operating characteristics to those of the fluorescent lamp.
2. Description of Prior Art
Inverter-type fluorescent lamp ballasts in which a series-resonant LC circuit is used for matching the inverter's operating characteristics to those of the fluorescent lamp (i.e., series-resonance-loaded inverter ballasts) have been described in published literature, such as in U.S. Pat. No. 3,710,177 to Richard Ward or U.S. Pat. No. 4,370,600 to Zoltan Zansky.
In these ballasts, the fluorescent lamp is typically connected in parallel with the tank-capacitor of the series-resonant LC circuit, and the high voltage developed across this tank-capacitor (due to so-called Q-multiplication) is used for starting and operating the fluorescent lamp.
However, if the load is removed from such a series-resonant LC circuit, which occurs whenever the lamp is removed therefrom or whenever the lamp fails to operate (as is bound to happen toward end of normal lamp life), the voltage developed across the tank-capacitor (and thereby the power drawn by the series-resonant circuit) will become so high as to cause circuit damage--except if some form of over-voltage or over-current protection has been provided.
While Ward does not describe any specific means of over-voltage protection, Zansky does provide for such means in the form of a voltage-clamping arrangement; which arrangement operates to limit the voltage across the tank-capacitor by way of using the inverter's DC supply-voltage as a load for the series-resonant LC circuit whenever the magnitude of the voltage across the tank-capacitor exceeds a certain pre-determined multiple of the magnitude of the DC supply-voltage. Of course, to permit proper lamp starting, it is necessary that this clamping-voltage limit be set higher than the worst-case lamp starting-voltage for the minimum anticipated DC supply-voltage.
In summary, although the best arrangement that can be found among all known prior art voltage-clamping circuits does indeed provide a degree of mitigation against over-voltage and excess power-draw as resulting from having an unloaded series-resonant LC circuit connected directly across the output of an inverter, it provides far from a fully satisfactory solution. Some of its more serious limitations are as follows:
(a) Although the typical prior art voltage-clamping arrangement significantly limits the net power drawn by the overall ballasting system in case of an inoperative lamp, the amount of power that has to be handled by the inverter itself has been limited to a much lesser degree: it still has to handle on a continuous basis all the power associated with having the voltage-clamping arrangement connected across the tank-capacitor--which voltage-clamping arrangement, as far as the inverter and the LC circuit are concerned, is just another load. This implies that the transistors of the inverter have to be able to handle on a continuous basis several times the amount of power that they have to handle in order to provide just for the continuous operation of the regular fluorescent lamp load. (Note: For a fixed input voltage to a series-resonant LC circuit, the power absorbed by a load connected across the tank-capacitor of the LC circuit is substantially proportional to the magnitude of the voltage developed across the tank-capacitor.)
(b) With the DC supply-voltage being used as the voltage-clamping means, the clamping-voltage limit is fully dependent upon the magnitude of this DC supply-voltage. Thus, in cases where the magnitude of the DC supply-voltage may change (as it invariably will in situations where the DC supply-voltage is derived from an ordinary electric utility power line), it is necessary to arrange for the clamping-voltage to be adequately high for proper lamp starting even at the lowest anticipated level of DC supply-voltage; which implies that the clamping-voltage will be that much higher for the maximum anticipated level of DC supply-voltage. Thus, the demands on the inverter in terms of power handling capabilities increase in proportion to the ratio of maximum-to-minimum levels of DC supply-voltage. (Or, conversely, there is a distinct limitation on the minimum level of DC supply-voltage for which the ballast will be able to start the fluorescent lamp).
(c) Even though the inverter in the ballast is able to handle on a continuous basis the excess current that results with an inoperative fluorescent lamp, there is a sufficient amount of wasted power associated with doing so; which implies that--in case such an inoperative fluorescent lamp is left connected with the ballast for an extended period of time, as in indeed apt to occur near end of lamp life--a significant amount of energy is wasted.
Of course, it would be possible to use an ordinary voltage clamping means (such as a Varistor or a Zener diode) for providing the requisite voltage-clamping effect; except that this would result in such a gross amount of excess power dissipation as to be non-feasible both from the viewpoint of size and cost of the voltage-clamping means itself as well as from the viewpoint of excessive energy waste.
Yet, this approach does have the advantage of providing the fluorescent lamp with a starting voltage of substantially constant magnitude regardless of relatively wide variations in the magnitude of the DC supply-voltage.
Thus, while there are several inherent and potentially important advantages associated with resonance-loaded inverter ballasts, the several limitations associated with such ballasts according to prior art are severe enough to prevent their widespread application.